To avoid thermal shorting, the MTPV system disclosed herein is preferably operated in a vacuum enclosure or housing which enables an evacuated gap and gap spacers to be employed to set the gap between the infrared radiation emitter and the photovoltaic cell receiver in a manner which minimizes heat transfer through the spacers. The gap spacers may be fabricated, for example, of silicon dioxide. Phonons or non-radiated energy carriers are a source of inefficiency. Unlike photons, phonons transfer energy from the source but they do not have the individual energy to excite electrons across the submicron gap.
As described in the above referenced paper, a method of forming the spacers used to set the gap between the hot infrared radiation emitter and the photovoltaic cell substrate is to grow a thick oxide on the infrared radiation emitter and form the oxide through such methods as photolithography and plasma etching into cylindrical spacers. These spacers which have a flange on an end are formed are formed in such a manner that the thermal path is about 15 microns long although the spacers only protrude less than a micron above the emitting surface. This allows setting a sub-micron gap while minimizing parasitic heat transfer through the spacers to the photovoltaic cell.
In the prior art, the back surface of the photovoltaic cell is used as the mating surface with the infrared radiation emitter to form the sub-micron gap. A disadvantage of this configuration is that it places stringent demands on the photovoltaic cell. For example, most photovoltaic cells are front illuminated and cannot readily be designed flat enough to permit a submicron gap to be formed with the surface. The prior art arrangement shown in FIG. 1 also precludes other options as will be described in enumerating the advantages of the present invention.
There is a need for a more facile, less complicated and less costly structure and method of fabrication of a submicron gap thermophotovoltaic device.